This is probably a waste of time as I suspect you already know the answer and are playing armchair lawyer rather than looking at the point being made, but I'll make it anyway.
Apple does not own 3rd party software yet they maintain 100% control of DISTRIBUTION OF ALL iOS SOFTWARE PRODUCTS. Now I know it's damn hard to comprehend that there is more than one aspect to modern day computing (hardware, software and distribution of said products) and that ALL THREE represent potential for abuse of market power. Namely, Apple says if you want to sell something like say, Microsoft Office for the iPhone, iPod Touch and iPad (all iOS computing devices), you MUST pay us 30% of all your sales. You are NOT allowed to sell your software for iOS devices using any other distribution methods or stores. Apple may own the hardware designs and Apple may own the operating system, but Apple should not have the right to force people to sell their products using their stores (whether virtual or brick and mortar). The same should apply to Android and any other operating systems. 3rd party software distribution should be the right of the authors, not Apple. Only the most narrow of vision people would still claim iOS is a "phone" operating system and thus not an actual computer market. You can't make that claim and then turn around and say the tablet (iPad) is the future of home computing as some have said on here.
Tying is the process of connecting separate markets and services together in such a way as to THWART COMPETITION for those additional profit making potentials. Apple ties software to hardware on ALL its devices (including Macs) thus denying 100% of all hardware competition for the OSX market (and has sued in court over this). Apple ties application distribution to iOS devices denying authors the right to distribute their software for their devices by means of their choosing (it would be like Chevy operating a network of gas stations and requiring you to fuel up at them instead of other gas stations if you buy a Chevy. That is EXACTLY what Apple does. Apple doesn't claim to own "gas" (3rd party software "Apps" in general), but does take 30% of all sales of "gas" for their products with no other gas stations being allowed to compete for use in Apple "cars" (iOS computer products).
Now the armchair lawyers can argue all day long on here about what is "legal" in modern day courts in America (many would now claim the Supreme Court no longer honors the Constitution and writes their own LAWS as they see fit these days so it may be MOOT what is "legal" and what is not when power is abused), but this is not an argument about what is legal. It's an argument about what is RIGHT. In Capitalism, competition is supposed to be the central motivating power and cornering markets and sub-markets to thwart alternate competition and distribution certainly violates the spirit of Capitalism if not the actual laws used to enforce it in a given country. So you can argue all day long about the meaning of the word "monopoly" in legal terms, but denying competition for the distribution of an entire market segment is an ABUSE of power in my opinion. Apple is no longer some tiny company that can't afford to play "fairly" in order to survive (as people have often made excuses for them in the past regarding things like not allowing OSX to run on 3rd party computers) but rather is the #1 tech company on the planet. If Apple cannot make money without abusing competition t hen there's something wrong with both Apple and the system period. App distribution is a tiny part of Apple's overall income and they should not have to force people to use iTunes and/or the App Store in order sell iPhones.
There is a real DIFFERENCE between HARDWARE and SOFTWARE. They are different market segments. Regardless, how would writing my own operating system (impossible for one person to do these days for anything modern) allow me to run OSX and its software on better hardware??? There wouldn't be ANY available software for such a system even if it were possible! The whole damn point is I want to run Mac software (and thus its OS as well) on better hardware than Apple will offer (forget the price; you can't even get what you want because Apple only believes in thin thin thin garbage anymore and doesn't update their Mac Pro but once or twice a decade these days).
In short, Apple should be forced by law to make their OS for sale for 3rd party products. I can see Europe doing something like this in the future as they seem to comprehend the problems of letting large companies control entire markets and market segments better than the greedmongering corporations using unlimited money to control the political process and therefore the government in the USA. In short, the US is becoming completely corrupt to special interests and large corporations and ignores what the people actually want.
Finally, any Mac "fan" knows that Microsoft Windows is not a great alternative to OSX and therefore having to switch entire operating systems and ALL one's software library over at GREAT COST still won't get you a better experience if you can't stand the operating system. I should not have to pick between the operating system and the hardware I can use it on, but that is the situation Apple puts us Mac users in every day of every week. And the only difference between OSX and iOS in these situations is that there are FAR MORE iOS users than Mac users out there and far more iTunes users than other stores and that is why the FTC's attention has been aroused. That doesn't make their OSX behaviors any more acceptable to some of us, but clearly numbers drive the FTC with their limited resources. You can state they have no case and maybe they do and maybe they don't (I don't trust the courts any more regardless), but clearly Apple is no longer invisible to the FTC nor should they be.
This statement tells me you don't appear to get how BAD it is for society (legal or not) for one or two companies to control EVERYTHING in a given market(s). It leads to stagnation and lack of creativity. It leads to the stock market being in situations where a few companies are "too big to fail" and can cause a global recession or worse. We've seen this on other fronts quite recently. Allowing companies to be "too successful" to the point of the consumer have few if any choices is bad. I have no choices when it comes to "cable". You can say that I can get satellite but they really aren't the same thing these days with services integrated like phone, TV and Internet). If cable has the only FAST Internet around, I have no choices if I need fast internet and that company can charge almost anything they want to the point where I'd have to MOVE to get something else. That isn't helpful for society or consumers to be in that situation. Now the cable companies are merging and moving is becoming a harder option as one company controls the cable in entire states or regions of states. DirectTV isn't very helpful for phone/Internet in most markets.
Hence my POINT isn't about legality once again, but about infrastructure and countries allowing a handful of companies to control access to important things like utilities and even air travel (as the number of airlines continues to shrink shrink and shrink some more) and that only leads to overtly high prices as they know they have you. It's why gas stations all charge the same or almost the same amount for gas. It's collusion and it's designed to rip off the consumer. Now maybe you prefer a world where companies can rip people off legally, but I do not.
Back to the original point. You can claim that Apple doesn't control streaming or even force people to subscribe on their site. But if they take ANY action against Spotify for telling their customers that they can save money by buying directly from them (e.g. by pulling their App from the App Store as a response), THEN I have a real problem as I did with them "punishing" Monster for filing an unrelated lawsuit (justice doesn't mean you can do more unrelated unjust things to try and stop justice from prevailing on the original problem). But at LEAST Spotify can try to distribute directly to the consumer for those with Apple devices. That is NOT true of "Apps" for Apple devices! You are forced to use the App Store (unless you want to count the tiny number of jailbroken devices out there which represent no significant market at all).
